1. Field of the Invention
The present invention relates to composite materials and processes for their use in the aerospace industry. More particularly, the invention is directed to quasi-isotropic chopped prepreg and the use of such composite material for the production of structural parts and other components having complex geometries that are designed for use in aircraft and other aerospace vehicles.
2. Description of Related Art
Lightweight metals, such as aluminum and magnesium alloys, have long been used as standard materials to make a wide variety of parts that are used in aircraft and other aerospace vehicles. Stronger and heavier metals, such as steel and titanium alloys, are also commonly used in place of aluminum to make aerospace components where there is a particular need for structural strength, fatigue resistance and/or galvanic corrosion protection.
Composite materials have become a popular replacement for many aerospace metal parts because composites are much lighter. However, there are a number of complex engineering issues that must be considered when replacing a metal part with a composite material. The performance of the composite part under a number of different stress loads must be carefully analyzed and compared to the metal part being replaced. Typical mechanical properties that are measured include compression strength, compression modulus, tensile strength, tensile modulus, flexure strength, flexure modulus and short beam shear. Standardized tests for measuring these various mechanical properties are described in ASTM D6272, ASTM D3039, ASTM D695 and ASTM D2344.
The flaw sensitivity properties of the composite part must also be carefully analyzed and compared to the metal part that is being replaced. This is especially important with respect to composite parts that include holes for attaching the part to aircraft structures. Such parts include window frames, wing fairing supports, flange supports, frame gussets, rudder actuator brackets, shear ties, seat pedestals, cargo floor flange supports, storage bin fittings, antenna supports, intercostals and other aerospace parts that form bolted or riveted joints in the vehicle structure. The holes form voids in the composite that are considered to be flaws. Exemplary flaw properties that must be analyzed include un-notched compression, open hole compression, bearing (ASTM 5961) and compression-after-impact.
Prepreg is used widely in the manufacture of composite parts. Prepreg is a combination of an uncured resin (matrix) and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network have a large affect on the structural properties of the part. Prepreg is a preferred material for use in manufacturing aerospace composite parts, since it is important that each composite part meet specific design tolerances that have been established for the part.
The fiber reinforcements that are commonly used in aerospace prepreg are multidirectional woven fabrics or a unidirectional tape that contains fibers extending parallel to each other. The fibers can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber reinforcement configurations are impregnated with a carefully controlled amount of uncured resin. The resulting prepreg is placed between protective layers and rolled up for storage or transport to the manufacturing facility.
Prepreg may also be in the form of short segments of chopped unidirectional tape that are randomly oriented to form a non-woven mat of chopped unidirectional tape. This type of pre-preg is referred to as a “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg is similar to the more traditional non-woven fiber prepreg, except that short lengths of chopped unidirectional tape (chips) are randomly oriented in the mat rather than chopped fibers. Quasi-isotropic chopped prepreg is considered to be “transversely isotropic”. The random orientation of the unidirectional chips provides isotropic properties in the plane of the mat. The quasi-isotropic chopped prepreg is therefore a transverse isotropic material. Properties are the same in any direction within the plane of the mat. Outside the plane of the mat (z direction), the properties are, however, different.
Quasi-isotropic chopped prepreg has been available commercially from Hexcel Corporation (Dublin, Calif.) under the tradename HexMC®. Quasi-isotropic chopped prepreg has been used in the past for a variety of purposes including bicycle parts and various other molded parts. However, quasi-isotropic chopped prepreg materials have not been used in the manufacture of aerospace parts. This is especially true for aerospace parts, such as aircraft window frames, that are bolted or riveted to the aerospace structure and a multitude of gussets, brackets and connectors, such as cargo floor flange supports, that form bolted or riveted joints in the vehicle structure.